This invention relates to the production of nanostructured or ultrafine particles (i.e. particles of less than 1 micron and preferably in the size range of 25-500 nm) of lithium titanate (Li4Ti5O12) a compound, which is an important anode material for rechargeable energy storage devices such as Li based batteries and asymmetric hybrid cells. The electrochemical performance of nanostructured lithium titanate exceeds that of coarse (large) size particles of the same material.
Nanomaterials-derived products are being actively pursued for use in a wide range of applications, including electrochemical energy storage and generation, chemical sensors, optoelectronics, semiconductors, wear and scratch resistant coatings, and heat transfer. The interest stems from the fact that researchers see immense potential for improving functional properties of components and devices by nanostructuring. In some cases, the use of nanoparticles as feedstock material can facilitate processing of an improved end product at a lower cost. However, while the use of nanoparticles as starting material can lead to benefits in a number of applications, researchers must tailor the structure and composition of the starting powder in order to maximize the property enhancements and performance and realize the true potential of nanomaterials.
Over the past several years, a number of techniques have been developed for the production of ceramic nanoparticles. These include: laser ablation, microwave plasma synthesis, precipitation from a solution, spray pyrolysis, plasma arc synthesis, hydrodynamic cavitation, and gas condensation using either a physical evaporative source or chemical precursors. Vapor phase processes are capable of producing well-defined spherical nanoparticles with narrow particle size distribution. Several single component oxides can be produced by an atmospheric flame process at low cost. However, it is extremely difficult to control the composition of multi-component ceramic powders because of the significant variations in the vapor pressures of different constituents. On the other hand, solution-based processes have an excellent control on the composition, but particle characteristics are not as good as those of produced by any of the vapor phase process. The synthesis method discussed in this patent application bridges the gap between liquid and vapor phase processing routes to produce nanostructured doped/multi-component ceramic powders with well-defined particle characteristics.
This invention relates to nanostructured (or ultrafine) Li4Ti5O12 powders with a spinel-type structure with improved Li-ion diffusion. Li4Ti5O12 can also be written as Li(Li0.33Ti1.66)O4. A spinel structure consists of eight subcells, and each subcell has four oxygen atoms, four octahedral interstices and eight tetrahedral interstices. In each elementary cell, two octahedral sites are filled with Li and Ti atoms in a ratio of 0.33: 1.66 and one tetrahedral site with one Li atom.
These materials are of particular interest as anode for Li-based rechargeable energy storage devices. The present invention includes nanostructured (or ultrafine) Li4Ti5O12 powders and method of making the same. The present invention relates to a general method for the production of nanostructured (or ultrafine) Li4Ti5O12 powders. Li4Ti5O12 is an attractive negative electrode material for secondary rechargeable energy storage devices wherein Li-ions are cycled in and out during a charging and a discharging process of the device. Three Li ions can be inserted into the structure according to the reaction:
Li4Ti5O12⇄Li7Ti5O12xe2x80x83xe2x80x83[1]
The reaction occurs at approximately 1.5V vs metallic lithium, thereby providing a relatively safe electrode system compared to carbon in which insertion of Li ions occurs at a voltage range of 0.0 to 0.5 with respect to Li. However, safety is gained at the expense of cell voltage and energy density. From a structural viewpoint, Li4Ti5O12 is an ideal anode for Li-based rechargeable batteries, because the Li insertion into the cubic Li4Ti5O12 spinel structure occurs without any change in the lattice parameter (8.36 xc3x85); thereby providing an extremely stable electrode structure. Negative electrodes made of Li4Ti5O12 material can undergo many hundreds of cycles without structural disintegration. Moreover, lithium insertion causes a first-order displacement of the tetrahedrally-coordinated Li ions in the Li4Ti5O12 spinel structure into octahedral sites to generate the ordered rock-salt phase Li7Ti5O12. The insertion (and extraction) of lithium is thus a two-phase reaction which provides a constant voltage response atxcx9c1.5V. Furthermore, the voltage of a Li/Li4+xTi5O12 cell changes abruptly at the end of discharge and charge. Thus, a Li4+xTi5O12 spinel electrode provides very sharp end-of-charge and end-of-discharge indicators which is useful for controlling cell operation and preventing overcharge and overdischarge.
Several of the commercial applications, including hybrid electric vehicles, uninterruptable power sources (UPS) and power tools, require that batteries be able to charge to their full capacity in a short period of time, less than 30 minutes. However, anodes made of micron-sized or larger Li4Ti5O12 spinel particles do not exhibit good rate capability because of poor Li-ion diffusion.
In the past, attempts have been made to improve the electronic conductivity of Li4Ti5O12 phase by substituting Li-ions on octahedral sites with Mg or Al. U.S. Pat. No. 6,221,531 to Vaughey et al. discloses electrically conductive Li[Ti1.67Li0.33xe2x88x92yMy]O4 (M=Al or Mg) powders. However, improving electronic conductivity of Li4Ti5O12 phase will not enhance its charging rate capabilities as the electronic conductivity of Li4Ti5O12 anode is important only during the discharging process. The present invention focuses on developing Li4Ti5O12 materials with high Li-ion diffusion capabilities, which implies high ionic conductivity, and hence high charge rate capabilities.